Pressure-sensitive, hot-melt adhesives are usually made up of two general types. In one type, the product to be affixed to an object, for example a label, is of a permanent nature. In the other, the label, originally affixed to an object, is expected to be peeled off at a later date. But, since there are many surfaces with different and distinct adhesive characteristics, supplying non-permanent pressure-sensitive, hot-melt adhesives can be a problem. Insufficient tack could lead to premature failure, while very aggressive adhesives would not be removable except by applying excessive force. The aggressive adhesives are not water releasable, and therefore cannot be peeled off by the application of water or by wetting, for example with a wet paper towel. An answer to this problem can be found in employing a pressure-sensitive hot-melt adhesive which can be removed with the aid of a wet paper product, for example.
As an illustration, if a permanent label on a piece of porcelain is to be removed, it might have to be scratched off. That could be a risky procedure, as the porcelain surface might be scratched and any records on the label destroyed in the process. On the other hand, a temporary label might fall off before the procelain object is sold, and thus the manufacturer's identification, directions, or seal would be lost. With a "wash-away", or water-soluble label, the adhesion can be strong yet, when desired, the label can be loosened and removed by wetting the label with, for example, a wet paper towel.
This novel adhesive product is a serendipitous result of work toward development of a remoistenable, non-blocking hot-melt adhesive with a non-bitter taste, as would be needed to seal an envelope.
In previous work attempting to achieve remoistenable hot-melt adhesives for envelopes, a remoistenable adhesive had been perfected, but the product suffered from an undesirable bitter taste. This bitter taste was due to such key ingredients as the rosin or tall oil acids present in the formula. Replacing this type of compound, for example, with its ester, cured one problem and caused another, namely, a poor bonding quality. Recently, it was found that high acid number organic compounds, such as free fatty acids, overcame this drawback without causing any loss of bonding characteristics. Thus it is possible to provide "lickable envelopes" according to the invention described in our copending application Ser. No. 200,392, filed on Oct. 24, 1980, and entitled "Water-activatable Hot-melt Adhesives". During this development work, it was noticed that the use of an excess of liquid fatty acids beyond the limits described in our above-noted copending application renders the adhesive pressure-sensitive. Thus, the possibility arose of making a water-soluble (peelable), pressure-sensitive, hot-melt adhesive.
Of course, the non-bitter qualities so necessary for envelopes were now no longer required, and the new adhesive can now include rosin products and optionally certain types of polyamides to improve the product.